1. Field of the Invention
The present invention relates to a method of enriching a combustible gas providing a gas introduction section to which a source gas is fed, a gas discharge section for discharging an off gas, a gas collection section for collecting a product gas, a plurality of adsorption towers each charged therein with an adsorbent for selectively adsorbing the combustible gas, a pressure equalization passage interconnecting the adsorption towers with each other, and a pressure equalization passage opening/closing valve incorporated within the pressure equalization passage; the method comprising:
(1) an adsorption step for feeding the source gas to the adsorption towers from the gas introduction section for causing the combustible gas contained in the source gas to be adsorbed to the adsorbent and causing un-adsorbed gas un-adsorbed to the adsorbent to be discharged from the gas discharge section;
(2) a desorption step for causing the combustible gas adsorbed to the adsorbent to be desorbed from the adsorbent and extracting the desorbed combustible gas to the outside via the gas collection section;
the above steps (1), (2) being effected in alternation; and
(3) a pressure equalization step effected between a pair of adsorption towers interconnected via the pressure equalization passage after the adsorption step in one adsorption tower and after the desorption step in the other adsorption tower, with setting open the pressure equalization passage opening/closing valve in the pressure equalization passage.
2. Description of Related Art
In general, a combustible gas has possibility of explosion. It is said that when a combustible gas is contained within a predetermined concentration range in e.g. a source gas, there exists possibility of its explosion. The concentration range having the explosion possibility differs, depending on the kind of the combustible gas. In general, however, it is believed that the explosive range is where the combustible gas is contained by from 5 to 20 Vol. % approximately. And, it is also believed that in the case of methane gas too, there exists possibility of its explosion in a similar concentration range.
Further, it is believed that the possibility of explosion of combustible gas relates not only to the concentration of the combustible gas, but also to presence of an oxygen gas in a predetermined concentration range. This concentration range is said to be where the oxygen gas is contained by 10 Vol. % or more.
Therefore, in handling a gas containing a combustible gas, sufficient caution must be taken about the above-described concentration ranges of the combustible gas and oxygen gas. In particular, in the case of a gas containing a combustible gas or an oxygen gas near the explosive concentration range, it is important to keep out of the above-described concentration range through adjustment of the concentration range of the combustible gas or oxygen gas.
Here, a methane gas after enrichment has a relatively high concentration (about 60 Vol. % methane concentration) which is outside the explosive concentration range. On the other hand, in the case of an offgas obtained by extracting a certain amount of methane from a coal mine gas (having a methane concentration of 44 Vol. % approximately and an oxygen gas concentration of 12 Vol. % approximately), this off gas contains a relatively low concentration of methane gas (methane gas concentration of 44 Vol. % or less). Further, the above off gas contains also a predetermined concentration (oxygen gas concentration of approximately 12 Vol. % or more). Thus, with this offgas, both the concentration of the methane gas and the concentration of oxygen gas may fall within the respective explosive ranges thereof. This therefore is problematic for the possibility of explosion of offgas.
Incidentally, in case enrichment is carried out with suitably using an adsorption tower accommodating therein an adsorbent for selectively adsorbing combustible gas, normally, in the adsorption step, a source gas is introduced to the gas introduction section and the rest of the gas after the adsorption is discharged through the gas discharge section which is provided on the opposite side away from the gas introduction section. The management of adsorption in this adsorption step is effected, with taking into account the concentration of the combustible gas in the off gas to be discharged to the outside, in connection with the relation with the explosion limit described above. For instance, in connection with the relationship with the amount of adsorbent held inside the adsorption tower, there will be obtained in advance an adsorption period which constrains the concentration of combustible gas discharged as off gas below the explosion limit. Then, as the adsorption is effected with reference to this adsorption period as the upper limit, a favorable operation condition will be maintained. Here, the inside of the adsorption tower after the adsorption step is under a pressurized state with the source gas fed thereto. Further, as the adsorbent adsorbs mainly the combustible gas, in the remaining space inside the adsorption tower, gases (mainly air) other than the combustible gas as impurities will be present.
Further, the combustible gas which was adsorbed in the adsorption tower will be taken out in the desorption step through the gas collection passage. However, gas that is desorbed from the adsorbent at an early stage in the desorption step contains other gases than the combustible gas, as impurities. Therefore, in order to obtain a combustible gas with high purity, this gas cannot be collected directly as a product gas. Rather, it is preferred that this gas be further purified for extracting the combustible gas therefrom. With this, the purity of the product gas can be enhanced and at the same time the collection ratio of combustible gas can be enhanced as well.
Meanwhile, considering now pressure variation inside each adsorption tower, the inside of an adsorption tower which has undergone an adsorption step is under a high-pressure condition. On the other hand, the inside of an adsorption tower which has undergone a desorption step is under an extremely depressurized condition due to completion of discharge of the combustible gas therefrom. Also, when another adsorption step is to be effected after completion of a desorption step, the pressure will be progressively increased from the above-described highly depressurized condition, thereby to prepare a condition that allows adsorption of the combustible gas, thus shifting to the operation for adsorbing combustible gas in the source gas.
Then, if a pressure equalization passage interconnecting adsorption towers is provided and a pressure equalization passage opening/closing valve is incorporated within this pressure equalization passage, when an adsorption tower which has completed the adsorption step (this adsorption tower will be referred to as “the first adsorption tower” hereinafter) is about to initiate a desorption step, the gas desorbed from the adsorbent (this gas will be referred to as “the early desorption gas” hereinafter) can be supplied to another adsorption tower which is about to initiate an adsorption step (this adsorption tower will be referred to as “the second adsorption tower” hereinafter). With this, the combustible gas contained in the early desorption gas can be supplied to the second adsorption tower to be used in the subsequent adsorption step to be effected therein. Therefore, in the first adsorption tower, the concentration of the combustible gas that is collected in the subsequent desorption step can be increased, whereas, in the second adsorption tower, the combustible gas can be collected from the early desorption gas with pressure increase required for shifting to the adsorption step (so-called pressure equalization step). (see Patent Document 1).